Acetic acid is produced industrially by carbonylating methanol in the presence of water, a rhodium catalyst, a metal iodide, and methyl iodide. In the methanol-carbonylation process, acetic acid is purified and productized by using process units including a reactor for carbonylating methanol under a carbon monoxide atmosphere, an evaporator for separating the reaction mixture fed from the reactor into a volatile phase and a less-volatile phase, a column (or light end column or splitter column) for distilling and separating the volatile phase into at least an overhead and an acetic acid stream, and a dehydration column for separating water from the acetic acid stream. If necessary, acetic acid is productized by further using a column (or heavy end column) for separating higher boiling point impurities and/or a production column following the dehydration column.
Regarding the methanol-carbonylation process, Japanese Patent Application Laid-Open Publication No. 2007-526310 (JP-2007-526310A, Patent Document 1) discloses an improved method for reducing and/or removing a permanganate reducing compound (PRC), a C3-8carboxylic acid, and a C2-12alkyl iodide compound; the method comprises distilling a volatile phase from a reaction mixture to form a first overhead, distilling the first overhead to form a second overhead containing methyl iodide, dimethyl ether, and the PRC, subjecting the second overhead to water extraction twice, and directly or indirectly introducing at least a portion of the resulting second raffinate to a reaction medium.
Unfortunately, in such a methanol-carbonylation process, corrosion may occur in process units and/or lines. Specifically, an inner wall of a process unit and/or line is selectively corroded, which may result in pitting corrosion or spot corrosion that forms pores. Moreover, a product acetic acid may be colored, lowering in quality.
In the methanol-carbonylation process, it is known that hydrogen iodide corrodes an inner wall of a process unit and/or line. Japanese Patent Application Laid-Open Publication No. 2009-501129 (JP-2009-501129A, Patent Document 2) discloses a process for producing acetic acid comprising: distilling an acetic acid stream containing acetic acid, hydrogen iodide, a lower boiling point component (or lower boiling component), and a higher boiling point component (or higher boiling component) in a first distillation column to form a first lower boiling point stream, a first higher boiling point stream, a first side-cut stream containing acetic acid; and distilling the first side-cut stream in a second distillation column to form a second lower boiling point stream, a second higher boiling point stream, and a second side-cut stream containing acetic acid; wherein water or water and at least one component (A) selected from the group consisting of methanol and methyl acetate is fed to the first distillation column to convert hydrogen iodide into a lower boiling component, such as methyl iodide, for removing hydrogen iodide.
Unfortunately, even after the separation of hydrogen iodide by such a process, pitting corrosion or spot corrosion may still occur in the process unit or line.